Whats the voltage suppost to be at the ballast risistor?

If you are sourcing the ignition on/run from the stock harness/switch, the pink wire under the dash from the ignition switch (which changes to the red w/green stripe wire I think) is a resistance wire, and drops approx 3v. So there would be 9v on the input of the ballast resistor, and 6v on the output side.

Ballast resistors are redundant if you are using the stock wiring, found that out hooking up a Mallory Unilite.

Wanted to bump this thread as it brings up a good question.

How many out there are not using a resistor between the ignition switch and coil?

I do not use one on the race car, but do have one on the daily driver Coupe?

Any bad stories about not having the resistor?

HistoricMustang
www.historicmustang.com

In my experiance, the coil will eventually die without the resistor.

I've seen it happen in less than a week (with daily driving).

I think the better question is why were coils ever designed to run on a lesser voltage anyway??

You need to check what your coil manufacturer says as far as if you need to run a ballist resistor or not. I'm running a Mallory ProMaster coil. I'm running a .8 ohm resistor because I'm also runing a Pertronix Ignitor (1st gen). The pertronix requires 1.5 ohms of resistence. The coil has a built in resistence of .7ohms, thus the need for the .8 ohm resistor. Now this is dictated by Pertronix, not Mallory. If I were running a Ford Duraspark ignition, then I wouldn't be using a resistor at all as it is not needed. Again, this is due to the distributor, not the coil.

Check with your coil manufacturer as YMMV.

[Edit]OK, what I said isn't totally correct. After reading Scott's post, I should have added that it depends on the type of ignition as well. For example, with points, I would need to run more ballast as I'd fry the points if I ran it with just 1.5ohms of resistence. How much, no idea. That's where your coil manufacturer will come in handy. Pertronix was the one that dictated how much ballast I had to use to prevent their unit from going up in smoke. I could run my ProMaster coil without any ballast and have no harm come to it. It's my Pertronix that will suffer. As Scott mentioned, you have 2 types of coils. The advantage of an externally resisted coil is that it allows you to use it with different ignition systems by adding or removing resistence to meet the requirements of your ignition. Conversely, using an internally resisted coil with 3.5ohms as Scott uses in his example below would be excessive for my Pertronix ignition, resulting in sub par performance. There, clear as mud.

Two basic coil types

Internally and externally reisisted.

Internally resisted coils will have aproximately 3.5 Ohms across the primary posts.

Externally resisted coils will have 1.5 Ohms or less resistance.

An externally resisted coil allows use of a shunt directly to battery positive on startup (in our case the "I" post on the starter solenoid). Mostly needed with points systems to help maintain a consistent coil saturaturation whether starting or running.

Electronic ignition (duraspark, HEI, etc), control this by varying the dwell constantly while monitoring coil saturation. Therefore an internally resisted coil is used to lower the voltage to a consistant state.

HTH,

Scott

Now i'm confused. I'm running the Duraspark dizzy and ignition box, along with an MSD Blaster coil. I'm still using the stock pink wire on one side of the coil and the other wire comes from the duraspark box. Did i need to wire up the ballast? MSD's website only deals with points and MSD boxes so they weren't much help.

The stock pink wire is a resistor

don't add in another one.
Scott

actually the red/green wire is the ballest wire. I asked pertronix, another mustang guy I know, + the Chilton repair book. The pink wire just carries the current the rest of the way to the coil without reducing it any further. On my pertronix coil it was required that I remove or bypass(in my case I bypassed) the resitor because it needed a 12v current to operate. I was dumb and tried with the ballest wire on and got lots of backfires through the carb and out the exhaust from high gas buildup.

66Runt said:

don't add in another one.
Scott

Click to expand...

Thats what i thought. The MSD instructions did say that i needed to add the ballast for either the points or the MSD ignition box and it still used the pink wire from what i remember.

A few years ago, when I was using a Pertronix Ignitor II and Flamethrower coil, I had no problems using the stock wiring to the coil.

When I upgraded to the MSD Ready-to-Run distributor and Blaster 2 coil, my motor had a high RPM miss when I took it to the track. So I bypassed the resistor wire so the coil and ignition and the miss was gone next time I went to the track.

I believe most aftermarket coils can handle a full 12 volts. In my car the resistor wire was Pink (the copper wire inside was really thin too) and had a cloth sheath around it.

A good "poor man's" check is to unhook the coil wire from the top of the distributor, place is 1/4" to 3/8" from a grounded metal source and have someone try to crank the car (of course it will not crank). You should get a good "blue" spark. If it is yellow the coil can handle more or it is going bad.

Caution - do not let the coil spark touch skin. If it does it will be one of those electircal shocks that you will never forget.

HistoricMustang
www.historicmustang.com

67coupe351w said:

...I think the better question is why were coils ever designed to run on a lesser voltage anyway??

Click to expand...

They're not, the idea is that you need a fatter spark to start the engine when it's cold, than to keep it running. If you've got a nominal 12V to start with, it wasn't easy before cheap electronics to get more volts to give a fatter spark, so you design the system to give a good spark for running the engine at, say 9V, then you got a full 12V to start the engine when you temprarily drop the ballast resistor out of circuit.

limey66 said:

They're not, the idea is that you need a fatter spark to start the engine when it's cold, than to keep it running. If you've got a nominal 12V to start with, it wasn't easy before cheap electronics to get more volts to give a fatter spark, so you design the system to give a good spark for running the engine at, say 9V, then you got a full 12V to start the engine when you temprarily drop the ballast resistor out of circuit.

Click to expand...

That's about it...

Except I'll add one thing... it's done more because while cranking the starter sucks so much power the battery is sucked down to only 9 or 10 volts. Not so much because you need a hotter spark while cranking.